Per- and polyfluoroalkyl substances (PFAS) are a class of man-made compounds that have been used to manufacture consumer products and industrial chemicals, including, inter alia, aqueous film forming foams (AFFFs). AFFFs have been the product of choice for firefighting at military and municipal fire training sites around the world. AFFFs have also been used extensively at oil and gas refineries for both fire training and firefighting exercises. AFFFs work by blanketing spilled oil/fuel, cooling the surface, and preventing re-ignition. PFAS in AFFFs have contaminated the groundwater at many of these sites and refineries, including more than 100 U.S. Air Force sites.
PFAS may be used as surface treatment/coatings in consumer products such as carpets, upholstery, stain resistant apparel, cookware, paper, packaging, and the like, and may also be found in chemicals used for chemical plating, electrolytes, lubricants, and the like, which may eventually end up in the water supply.
PFAS are bio-accumulative in wildlife and humans because they typically remain in the body for extended periods of time. Laboratory PFAS exposure studies on animals have shown problems with growth and development, reproduction, and liver damage. In 2016, the U.S. Environmental Protection Agency (EPA) issued the following health advisories (HAs) for perfluorooctanesulfonic acid (PFOS) and perfluorooctanoic acid (PFOA): 0.07 μg/L for both the individual constituents and the sum of PFOS and PFOA concentrations, respectively. Additionally, PFAS are highly water soluble in water, result in large, dilute plumes, and have a low volatility.
PFAS are very difficult to treat largely because they are extremely stable compounds which include carbon-fluorine bonds. Carbon-fluorine bonds are the strongest known bonds in nature and are highly resistant to breakdown.
The vast majority of available conventional water treatment systems and methods to remove PFAS from water have proven to be ineffective. See e.g., Rahman, et al., Behaviour and Fate of Perfluoroalkyl and Polyfluoroalkyl Substances (PFASs) in Drinking Water Treatment, Water Research 50, pp. 318-340 (2014), incorporated by reference herein. Conventional activated carbon adsorption system and methods to remove PFAS from water have shown to be somewhat effective on the longer-chain PFAS, but have difficulty in removing branched and shorter chain compounds, see e.g. Dudley, Master's Thesis: Removal of Perfluorinated Compounds by Powdered Activated Carbon, Superfine Powdered Activated Carbon, and Anion Exchange Resins, North Carolina State University (2012), incorporated by reference herein.
Appleman et al., Treatment of Poly-and Perfluoroalkly Substances in U.S. Full-Scale Treatment Systems, Water Research 51, pp. 246-255 (2014), incorporated by reference herein, reported that, similar to activated carbon, some conventional anion exchange resins may be more effective at treating longer chain PFAS than the shorter chain compounds. Other conventional anion exchange resins have shown some success in removing a broader range of PFAS, including the shorter-chain compounds, see e.g., Dudley, cited supra.
Conventional anion exchange treatment systems and methods typically utilize anion exchange resin where positively charged anion exchange resin beads are disposed in a lead vessel which receives a flow of water contaminated with anionic contaminants, such as PFAS. The negatively charged contaminants are trapped by the positively charged resin beads and clean water flows out of the lead anion exchange vessel into a lag vessel, also containing anion exchange resin beads. A sample tap is frequently used to determine when the majority of the anion exchange beads in the lead exchange vessel have become saturated with contaminants. When saturation of the resin anion exchange beads is approached, a level of contaminants will be detected in the effluent tap. When this happens, the lead vessel is taken off line and the contaminated water continues flowing to the lag vessel which now becomes the lead vessel. The lead-lag vessel configuration ensures that a high level of treatment is maintained at all times.
As discussed above, some conventional anion exchange resins can also be used to remove PFAS from water. A number of known methods exist to regenerate the anion exchange beads in the anion exchange vessel. Some known methods rely on flushing the resin with a brine or caustic solution. Other known methods may include the addition of solvents, such as methanol or ethanol, to enhance the removal of the PFAS trapped on the anion exchange beads. Effective resin regeneration has been demonstrated by passing a solvent (e.g., methanol or ethanol), blended with a sodium chloride or sodium hydroxide solution, through the resin. See e.g., Deng et al., Removal of Perfluorooctane Sulfonate from Wastewater by Anion Exchange Resins: Effects of Resin Properties and Solution Chemistry, Water Research 44, pp. 5188-5195 (2010) and Chularueangaksom et al., Regeneration and Reusability of Anion Exchange Resin Used in Perfluorooctane Sulfonate Removal by Batch Experiments, Journal of Applied Polymer Science, 10.1002, pp. 884-890 (2013), both incorporated by reference herein. However, such methods may generate a large amount of toxic regenerant solution which must be disposed of at significant expense.
Du et al., Adsorption Behavior and Mechanism of Perfluorinated Compounds on Various Adsorbents—A Review, J. Haz. Mat. 274, pp. 443-454 (2014), incorporated by reference herein, discloses a need to further treat the waste regenerant solution to concentrate the PFAS and reduce the volume of waste. This is a key step, because resin regeneration produces a significant volume of toxic waste.
The known methods for removing PFAS from water discussed above typically do not optimize the anion exchange resin and may have limited capacity for removing PFAS mass. Such known methods may also incompletely regenerate the anion exchange resin by attempting to desorb the PEAS from the resin. Such known methods may incompletely regenerate the anion exchange resin which may lead to a loss of capacity, otherwise known as active sites, during each successive loading and regeneration cycle. This cumulative buildup of PFAS on the ion exchange resin is often referenced to as a “heel,” and results in reduced treatment effectiveness as the heel builds up over time. Such known methods may also not reclaim and reuse the spent regenerant solution which may increase the amount spent regenerant solution with removed PFAS therein. This increases the amount of toxic spent regenerant solution with PFAS, which must be disposed of at significant expense.
Conventional systems and methods for attempting to remove PFAS also include biological treatment, air stripping, reverse osmosis, and advanced oxidation. All of these conventional techniques are ineffective and/or extremely expensive.